Method for improving hot strip mill processing

ABSTRACT

A system for processing steel strips in a hot strip mill includes an apparatus and method for removing the oxide layer in the finishing mill process such that the final thickness of the oxide layer is much less than previously possible. In order to reduce the thickness of the oxide layer, the invention provides for the removal of oxide scales from the surface of the steel strips at a distance in front of the working rolls of the finishing mill that minimizes the time the strip is exposed to ambient conditions after it is descaled and before it is received between the pair of working rolls. By minimizing this exposure time, the thickness of the oxide layers in the finished strips is substantially reduced.

This is a divisional application of application Ser. No. 910,503, filedSept. 22, 1986, now U.S. Pat. No. 4,796,450, issued Jan. 10, 1989.

DESCRIPTION OF THE INVENTION

This invention generally relates to hot strip mill processing ofelongated steel strips and, more particularly, to an improved apparatusand method for guiding and cleaning the elongated strips prior to theentering of the strips to the finishing mill operation.

In a hot strip mill, relatively wide steel slabs are rolled into verythin strips. Typically, a hot strip mill operation consists of threestages: a roughing stage, a finishing stage and a coiling stage. In theroughing stage, the thickness of the slab is reduced from a typicaldimension of 8-10 inches to 11/4 inches. The finishing mill receives thesteel strips after they have been reduced in thickness by the roughingmill, and the finishing mill further reduces the thickness of the stripsto approximately 0.060 inch to 0.500 inch thick. Typically, a finishingmill consists of approximately four to seven closely spaced rollingmills. From the finishing mill, the strips are coiled for convenience ofhandling and further processing of the steel strip.

In existing mills, the finishing process operates satisfactorily onlyunder normal conditions. When faced with an abnormal condition, such asa cambered strip, the finishing mill may jam because of cobbling of thestrip or the cambered strip may be slowed by frictional engagement withcentering guides, thereby slowing the speed of the strip. Slowing of thestrips may cause serious damage to the finishing mill since unacceptablecooling may occur prior to entry of the strips between a pair of workrolls. Too much cooling makes the strips too hard for the working rollsto compress and, as a result, the mill may (1) suffer a power overload,(2) create non-uniform thickness along the lengths of the strip, or (3)actually break the work rolls.

Much of the cooling of the strips prior to their entry between the pairof work rolls is a result of the effect of high pressure liquid(typically water) sprayed onto the strips in order to remove any buildupof oxides on the surface of the strips prior to their entry into theworking rolls. This removal of the oxide layer is commonly referred toas "descaling".

Because ambient conditions of the strips expose them to air, ferrousoxide layers are constantly forming on the surface of the strips.Therefore, after descaling, but before entry of the strips into theworking rolls, a ferrous oxide layer inevitably forms. Typically, apickling process employing acid is used to remove the oxide layers fromthe finished strips. Presently, the thickness of the oxide layers duringthe process is determined by the restrictions and placement of thedescaling mechanism. Reduction of the oxide layers formed during thefinishing process would result in savings in the pickle acid process.Moreover, replacement of the present descaling mechanism may reduceformation of oxides of relatively greater hardness (e.g., 1030 Vickershardness for Fe₂ O₃) By limiting the oxide formation to softer oxides,the wear of the working rolls could be dramatically decreased, therebylengthening the time period between shutdowns of the mill for replacingworn rolls.

In view of the foregoing, it is a general object of the invention toprovide an improved apparatus and method for guiding the steel strips asthey enter the finishing mill such that the descaling process can belocated at a minimum distance from the working rolls in order tominimize the amount of ferrous oxide buildup. In this connection, it isalso an object of the invention to provide an apparatus for reducing thetravel time of the steel strips between the area of descaling and thework rolls.

It is a further object of the invention to reduce the amount of acidnecessary to remove oxide layers from finished steel strips and toreduce the maintenance required for the work rolls of the finishingmill.

It is a more particular object of the invention to stabilize thevelocity and temperature of the strip as it enters the finishing mill tothereby improve the operation and reduce the iron oxide thickness.

Other objects and advantages of the invention will become apparent fromreading the following detailed description and upon reference to thedrawings, in which:

FIG. 1 is a schematic side view of a prior art finishing mill includinga descaler and a strip guide;

FIG. 2 is a block diagram of a finishing mill according to theinvention, illustrating the various input and output signals requiredfor guidance of the steel strips in accordance with the invention;

FIG. 3 is a side view of a schematic representation of an apparatussuitable for implementing a finishing mill according to the invention;

FIG. 4 is a plan view of the finishing mill of FIG. 3 taken along theline X--X in FIG. 3;

FIG. 5 is a graph illustrating the reduced oxide layer that may beexpected to be found on strips finished by finishing mills according tothe invention; and

FIGS. 6a and 6b are top and side views, respectively, of a crop shearguide for centering the strips according to a preferred embodiment ofthe invention.

While the invention will be described in connection with preferredembodiment, it will be understood that the following description is notintended to limit the invention to a particular embodiment. On thecontrary, it is applicant's intention to cover all alternatives andequivalents as may be included within the spirit and scope of theinvention.

Referring generally to FIGS. 1 and 2 of the drawings, in the finishingstage process of hot strip mills, a series of aligned delay tables 10receive an elongated steel strip 12 as it exits from the last roughingmill 13. Following the entry of the strip 12 onto the delay table 10,the head end of the strip is aligned and cut by a crop shear and guideassembly 15. The crop shear and guide assembly 15 is comprised of cropshears 15a and centering guides 15b.

Typically, a timing and control system 17 is provided which receivescontrol signals that automatically correlate the rotation of the cropshears 15a with the position of each strip 12 such that the head andtail ends are properly cut. The crop shears 15a comprise a pair ofopposing rollers (shown in FIG. 6b), each having a shear knife attachedalong the length of the perimeter of the rollers so that as the rollersturn about their horizontal axis in response to the timing and controlsystem 17, the shear knives meet and shear the strip 12. From the cropshears 15a, the strips 12 are carried by a roll-out table 55 to thefirst finishing mill 27 (FIG. 1). A series of mills or rolling stationsfollow the first mill as generally indicated by block 57 in FIG. 2.

Referring more particularly to FIG. 1, from the crop shears 15a, thesteel strips 12 enter a descaler 19 that includes pairs of upstream anddownstream damming rolls 21 and 23, respectively. Between the pairs ofdamming rolls 21 and 23, each strip 12 is exposed to a high velocityliquid that impacts upon the surface area of the strip in order toremove an oxide layer before the strip enters a pair of work rolls 25 ofthe first finishing mill F₁. In order to direct the high velocity fluid(usually water) onto the surface of each strip 12, a plurality of sprayheads 29a-d are positioned above and below the plane of travel of thestrip in order to ensure that both the top and bottom surfaces of eachstrip are descaled.

The damming rolls 21 and 23 function to limit the upstream anddownstream travel of the descaling fluid. In order to remove the fluidfrom the area between the pair of damming rolls 21 and 23, a hood (notshown) is typically provided for receiving the descaling fluid (and theoxide scales mixed therewith) after the fluid has impacted on thesurface of the strips, stripped away the oxide layer and deflected awayfrom the surfaces. Although the pairs of damming rolls 21 and 23function adequately to prevent upstream and downstream spreading of thedescaling fluid, they operate in a very poor environment of hot oxidescales and water, and therefore, require frequent maintenance andreplacement. Because the damming rolls 21 and 23 deteriorate rapidly,any neglectful maintenance may result in the damming rolls 21 and 23operating past their useful life and thereby causing improper processingof the strips such as scratches and rolled-in scales on the surface ofthe strips. In severe cases, the strip may even be stopped by thedeteriorated damming rolls 21 and 23, requiring the mill to be stoppedso the strip can be removed and scrapped.

From the descaler 19, the strips 12 are positioned by an entry guide 31prior to entering between the pair of work rolls 25. The entry guide 31supports each strip as it leaves the last pair of damming rolls 23 andalso serves to guide the strip toward the center line of the finishingmill F₁. Often a cambered strip 12 will come out of the damming rollpair 23 and hit the entry guide 31. Sometimes the strip 12 may hesitatebriefly, and then enter the mill with a cold band across the width ofthe strip caused by the cooling effect of the descaling fluid. Thesevery serious deviations from the normal operating mode of the mill mayresult in breaking the work rolls 25 or, less seriously, a temporaryshutdown of the mill while a cobbled strip 12 is removed.

In a conventional manner, top and bottom backup rolls 35 and 37 aremounted above and below, respectively, the pair of work rolls 25. Eachof the backup rolls 35 and 37 and the work rolls 25 include longitudinalshafts (not shown) that are journaled into opposing sides of the frame33 of the mill F₁. In order to cool the pair of work rolls 25, pairs ofupper and lower spray headers 39a,b and 41a,b are mounted adjacent thetop and bottom work rolls 25a and 25b, respectively. As a further aid incooling the work rolls, a top entry wiper 43 is provided for scrapingthe surface of the top work roll 25a. As a complement to the entry wiper43, an exit stripper 45 is mounted to the frame 33 and positioned toscrape the top work roll 25a on the exit side of the mill F₁. Similarly,the bottom work roll 25b is supplied with an exit stripper 47.

As indicated by the position of the entry guide 31 illustrated inphantom line, the design of the finishing mill F₁. must accommodate easyremoval and replacement of the pair of work rolls 25. Such accommodationis an important design consideration since the work rolls 25 requirefrequent replacement, and a design allowing for efficient replacement ofthe rolls minimizes the down time of the mill. Therefore, the distancefrom the last pair of damming rolls 23 to the pair of work rolls 25 isdetermined by the amount of room needed to remove the entry guide 31from its operating position proximate to the pair of work rolls so thatthe pair of work rolls can be easily removed from the mill housing 33for replacement or maintenance.

Typically, the entire operation from exiting the crop shears 15a toentry between the pair of working rolls 25 covers a distance ofapproximately 25 to 40 feet and occurs in about 8 to 15 seconds(assuming a velocity of 120 to 180 feet per minute). It is generallyaccepted that the distance from the pair of finishing work rolls 25 tothe crop shear 15a is determined by the design of the finishing millentry guide 31, and the amount of room needed to remove the entry guidefrom its position adjacent the work rolls. In order to remove the entryguide 31, there must be adequate distance between the last set ofdamming rolls 23 and the pair of work rolls 25.

Referring briefly to FIG. 5, the thickness of the oxide layer formed onthe surface of each strip 12 is a function of time and temperaturewherein the initiation of the oxidation process begins after each striphas been descaled and continues until the strip has sufficiently cooledto a temperature that will not support further, rapid oxidation. Asindicated by the exemplary plot A in FIG. 5 for the thickness of theoxidation layer in a strip 12 processed in a prior art finishing millsuch as that shown in FIG. 1, a significant amount of the growth of theoxide layer occurs between the descaler 19 and the work rolls 25. Uponentry of each steel strip 12 to the first pair of work rolls 25, thethickness of the oxide layer is compressed as indicated by the firstvertical line F₁ of plot A in FIG. 5. The following vertical lines inthe exemplary prior art plot A represent further compression of theoxide layer in the mill stations (F₂, F₃ etc.) that follow the firstpair of work rolls 25 in FIG. 1. Between each pair of work rolls, theoxide layer continues to thicken as indicated by the sloped portion ofthe plot A connecting the vertical line portions.

By reducing the time of exposure to ambient air after the strips 12 haveleft the descaler 19 and prior to their entry into the work rolls 25, asignificant decrease in the thickness of the oxide layer in the finishedstrip will result. Referring to the second exemplary plot B in FIG. 5,by decreasing the time of exposure for the strips 12 between thedescaler 19 and the work rolls 25, the thickness of the oxide layer whenthe strips enter the F₁ mill is much less than expected to be found instrips processed by the mill F₁ shown in FIG. 1. From this startingpoint of decreased oxidation, the same oxidation rates between thefollowing finishing stations F₂, F₃ etc. will result in finished steelstrips that have an oxide thickness that is considerably less thanpreviously possible.

There are at least two mechanisms at work which promote adisproportionately high rate of oxidation during the time following thedescaling of the strips 12. First, the temperature of the strips 12 isat its greatest prior to the introduction of the strips to the firststation of the finishing mill. It is commonly recognized that iron oxideformation, like most other chemical reactions, is accelerated withincreased temperature. Secondly, the time of exposure for the strips 12as they travel from the descaler 19 to the F₁ mill is the longestexposure between any individual mill in the finishing mill, resulting inthe relatively extreme build up of oxide thickness in front of the F₁mill as illustrated by plot A in FIG. 5.

It has also been found that the iron oxide formed on the surface of thesteel strips 12 form deposits of varying hardness. Specifically,deposits comprising FeO have a Vickers hardness between approximately270 to 350, deposits of Fe₃ O₄ have a hardness rating betweenapproximately 450 to 500 and deposits of Fe₂ O₃ have a hardness ratingof approximately 1030. It is believed that when the time of exposure ofthe steel strips 12 is limited between the descaling process and thefirst pair of work rolls 25, a significant lesser amount of the harderiron oxide deposits are formed. It is readily seen that reduction in theamount of harder iron oxide deposits formed on the steel strips 12 mayplay a major roll in minimizing the maintenance required for the workrolls.

In accordance with one important aspect of the invention, the descalingspray is directed to the surfaces of the strips 12 such that fluidcomprising the spray impacts onto the surfaces, strips the oxide layerfrom the surfaces and deflects upstream of the movement of the strip,carrying the oxide scales with it. By directing the descaling in amanner to ensure virtually all of the deflected fluid and scales aredeflected upstream, a descaling apparatus 70 does not require dammingdownstream flow of fluid and scales. Therefore, any space requirementsof the descaler 70 that are downstream of the spraying area areminimized. In accordance with another important aspect of the invention,an entry guide 53 (FIG. 3) is provided upstream of the descaler 70 sothat the descaler immediately precedes the work rolls 25 such that thedistance X in FIG. 2 may be minimized.

In keeping with the invention, a conventional hot metal detector 51(FIG. 2) is located downstream of a pair of entry guide rollers 53 suchthat detection of the hot steel strips 12 by the hot metal detectorprovides detection signals to the timing and control system 17 thatallow the system to synchronize changes in the position of the entryguide rollers with the movement of each strip. Similarly, a hot metaldetector 59 provides location information to the crop shears 15a and thecrop shear guides 15b. The entry guide rollers 53 preferably comprise apair of rollers mounted for rotation about vertical axes, and eachroller is mounted on an opposite side of the delay table 55 such thatthey form a channel that receives the strips 12 and functions totransversely position the strips prior to their entry into the workrolls 25 of the finishing mill F₁. Control of the movement of the entryguide rollers 53 and of the crop shear guides 15b will be discussed ingreater detail in connection with FIGS. 4 and 6a,b, respectively.

Referring to FIG. 3, a particular embodiment of the invention isillustrated for implementing the entry guide rollers 53 and descaler 70of FIG. 2. Where elements in the finishing mill F₁ of FIG. 3 correspondto like elements in the finishing mill F₁ of FIG. 1, they have beennumbered the same. Accordingly, these like-numbered elements will not bediscussed again in connection with FIG. 3.

The descaler 70 includes a descaling hood 63 pivotally mounted over thetop of the entry guide rollers 53 and having an opening downstream ofthe rollers for receiving descaling fluid deflected from the top surfaceof the strips 12. In order to aid the hood 63 in preventing the upstreammovement of the deflecting fluid and scales, an elastomeric flap 63a issecured to the opening of the hood 63 and is positioned so that in itsrelaxed state it extends into the plane of the motion of the strips 12such that when it is deflected by the oncoming head end of a strip itpositively engages with the top of the strip in order to provide a sealbetween the strip and the hood.

Because the entry guide rollers 53 are mounted for rotation andtransverse movement, they require protection from the high velocityfluid descaling headers 65 and 67 that function to descale the strips12. In order to provide such protection, a bottom deflector 69 ismounted to the forward frame member 33 of the finishing mill F₁ suchthat in its relaxed position (indicated by the phantom line outline69'), the uppermost portion of the deflector extends into the plane ofthe strips 12 so that the top of the deflector overlaps with the flap63a of the hood 63 in a direction orthogonal to the plane of the strips.By providing such an overlap, the fluid from the descaling headers 65and 67 is effectively deflected away from the entry guide rollers 53.

Movement of the deflector 69 from its position shown as 69' to itsposition shown in solid line is accomplished by the reaction of thedeflector to the impact of the head end of each strip 12 onto the tip ofthe deflector extending into the plane of the strip's motion. The thirdposition of the deflector 69 (indicated in phantom line as position 69")allows the deflector to be moved away from the operating area of themill so that sufficient room can be created to easily remove a cobbledstrip 12.

A top wiper 71 is mounted to the forward frame member 33 of the mill 61so as to be capable of movement between an operating position (shown insolid line) and a position for removing the pair of work rolls 25 shownin phantom. In order to guide the head end of each strip 12 to the areabetween the work rolls 25, a bottom guide 73 is provided. As with thetop wiper 71, the bottom guide 73 may be moved to an out-of-the-wayposition as illustrated in phantom line so that the work rolls 25 can beeasily removed.

As water is sprayed from the descaling headers 65 and 67 below and abovethe steel strips 12, respectively, it is deflected off the strips andcollected by the descaling hood 63 or directed downwardly by the bottomdeflector 69. From the descaling hood 63 the contaminated fluid iscollected in a trough portion 63b of the hood and directed to a flume(not shown).

In normal operation, the hood 63 is positioned by a cylinder 74.Movement of the bottom deflector 69 between its position 69' and itsposition shown in solid line is provided by a counterweight portion 69athat causes the deflector to naturally assume its position 69'. When astrip 12 is present, the deflector 69 is biased against the lowersurface of the strip. To move the deflector 69 into its position 69",conventional mechanical gearing may be used to move it and stabilize itinto that position. Bottom guide 73 is also provided with acounterbalance 73a that eases the rotation of the guide about its pivotpoint. But, unlike the deflector 69, the guide 73 is fixed in itssolid-line position unless rotated to its phantom-line position bysuitable gearing (not shown).

In keeping with the invention, the guide rolls 53a and 53b shown in FIG.4, are mounted for rotation about a vertical axis on a platform 75. Eachof the guide rolls 53a and 53b are mounted directly to a base section 77that in turn is mounted to the platform 75. A threaded shaft 79 spansthe length of the platform 75 and passes through bore sections (notshown) in the base sections 77. The threading of the shaft 79 is dividedinto two sections having reverse threading with respect to one another.Rotation of the shaft 79 by a motor and gearing unit 81 causes the baseunits 77 and the guide rollers 53a and 53b mounted thereon to move awayor toward the center line of the finishing mill F₁. In order to supportthe head end of each strip 12 as it leaves the delay table 55 and entersthe channel formed by the guide rollers 53a and 53b, a centering guide83 is located along the center line of the mill F₁ and spans the widthof the platform 75 so that the head ends of the strips 12 are supportedas they leave the roll-out table and prepare to enter between the workrolls 25.

In response to detection of the head end of a strip 12 by hot metaldetector 51 (shown FIG. 2), the timing and control system 17 suppliescontrol signals to the motor and gearing 81 so that the guide rolls53a,b are moved from their phantom-line positions to the solid linepositions after the head end of the strip has passed downstream of thechannel formed by the roll. By providing such movement of the rolls53a,b in coordination with movement of the strips 12, the head end whichis sometimes wider than the body of the strip may pass through a widechannel and the body of the strip may be centered by the narrowerchannel formed when the rolls are moved inwardly toward the central lineof the mill F₁.

In keeping with the invention, it is very important that the angles θ₁and θ₂ of the descaling spray with respect to the top and bottomsurfaces, respectively, of the strips 12 be adjusted to minimizedownstream flow. Normally, top and bottom spraying angles θ₁, and θ₂ of10 degrees to 15 degrees from perpendicular are used in the system ofFIG. 1. Applicant believes the best angle for θ₁, θ₂ (FIG. 3) is 25degrees to 30 degrees. Increasing the angle of impact of the fluidrequires the headers 65 and 67 to be moved closer to the strip 12surfaces than previously necessary. The closer the headers 65,67 to thestrips 12, the greater the angles θ₁ and θ₂ must be; thus, morecertainly ensuring no fluid flows downstream. But placement of theheaders 65,67 too close to the strips 12 may cause damage to the headersor sprays when a mill cobble occurs.

An angle of 25 degrees to 30 degrees appears to be a good compromisebetween the incompatible goals of maximizing the descalingeffectiveness, avoiding significant shortening of header life, andensuring all fluid and scale deflects upstream. Preferably, the spraypattern of the descaling headers 65 and 67 is such that the coolingcaused by the fluid spray is evenly distributed across the width of thestrip 12. In FIG. 4, the nozzles 87 of the descaler header 65 arearranged in two offset rows such that the spray pattern from a nozzle isapproximately equal to the distance separating adjacent nozzles in itsrow.

In order to aid the roller guides 53a,b in centering the strips 12, animproved crop shear guide is provided as illustrated in FIGS. 6a and 6b.Upstream of the crop shear rollers 15a are two opposing guide arms 91aand 91b that are each pivotally mounted to the delay table 10 at a firstend. At the second end of each guide 91a or 91b, a roller 91c shown inFIG. 6b is mounted for vertical rotation and engagement with the sidesof the strips 12. The second end of the guides 91a and 91b are mountedon base units 93a and 93b, and they include conventional lost-motiondevices (not shown) which accommodate the arcuate movement of the secondend of each guide as it rotates about its pivot point.

The crop shear guides 15b aid in centering the strips 12 as they traveldownstream on delay table 55 in a manner that complements the guidingfunction of the roller guides 53a,b, thereby allowing the roller guidesto be located upstream of the descaler 70 without sacrificing adequatecentering of the strips before they enter the working rolls 25.

In keeping with the invention, the crop shear guides 91a and 91b aresequentially moved to three distinct positions in response to passage ofthe strips 12 past the hot metal detector 59 (FIG. 2) such that thestrips are more closely centered on the delay tables 10 and 55 so thatthe entry guide rollers 53 may be successfully placed before thedescaler 70 without compromising the overall centering of the apparatusimmediately preceding the finishing mill.

Each of the base units 93a and 93b is mounted on a platform 95 spanningthe width of the delay table 10, and the base units include a commonthreaded shaft 96 similar to that provided in connection with the rollerguides 53. Also in a similar manner as the roller guides 53, a motor andgearing unit 97 rotates the shaft 96 in response to signals from thetiming and control system 17 (FIG. 2) so as to move the second ends ofthe guides 91a,b between first, second and third positions in responseto movement of the strips 12 detected by the hot metal detector 59 (FIG.2).

Specifically, as the tail end of a previous strip 12 clears the channelformed by the crop shear guides 91a,b (15b in FIG. 2), the timing andcontrol system 17 responds to the hot metal detector 59 by actuating thecrop shears 15a so as to cut the tail end of the strip. This same signalmay cause the motor and gearing 97 to open the channel formed by theguides 91a and 91b to an entry position shown in solid line as position"E" in FIG. 6a. As the head end of the next strip 12 passes under thehot metal detector 59, the timing and control system 17 causes the motorand gearing 97 to close the channel formed by the guides 91a,b as thehead end enters the crop shears 15a. The guides 91a,b are moved to thephantom line position indicated as position "I". In this position, thechannel formed by the guides 91a,b is as close to the width of the strip12 as possible without pinching the strip between the rollers 91c. Thesignal delivered to the crop shears 15a for cutting the head end of thestrip 12 also serves to generate signals from the timing and controlsystem 17 that cause the motor and gearing 97 to move the crop shearguides 15b to a third position shown in phantom line in FIG. 6a andindicated as position "0".

The centering action of the guides 15b in position "I" places the headend of the strip 12 in the center of the delay tables 10 and 55, andmovement of the guides 15b to form a wider channel in position "0"eliminates any force being put on a cambered strip 12. Therefore, thestrip 12 will be cropped by the crop shears 15a and proceed to the firstfinishing mill 27 in the exact center of the F₁ mill entry guides 53.After the strip 12 enters the work rolls 25, an electrical load signalcan detect the presence of the strip and serve to activate the cropshear guides 91a,b to return them to the position "I" in FIG. 6a. Inthis position, if the strip 12 is off center, the guides 91a,b willforce the strip 12 to center and maintain the strip in a centralposition during the entire rolling process.

Referring more specifically to FIG. 6a, the guides 91a, 91b are inposition "E" as the head end of a strip 12 approaches. The "E" positiongenerally centers the strip 12, but it allows enough of a channel widthto allow the head of the strip to proceed without stopping. After thehead end of the strip 12 is detected by the hot metal detector 59, theguides 91a, 91b are moved from the "E" position to their "I" position inorder to center the head end exactly on the center line of the F₁ millentry guides 53. The guides 91a, 91b stay in the "I" position onlymomentarily (e.g., 0.2 seconds) and are thereafter quickly moved to the"0" position. In the "0" position the guides 91a, 91b will not cause thestrip 12 to move off center if the strip is cambered. Once a load signalfrom the F₁ mill indicates the strip 12 has entered the mill, the guides91a, 91b are returned to the "I" position in order to keep the strip 12centered for the remainder of the rolling by the finishing mill.

From the foregoing, it will be appreciated that an improved method forguiding and cleaning steel strips formed in a hot strip mill is providedwhich can be installed in existing arrangements by replacing thestandard damming rolls 38 and installing the new system of a descalinghood 63 and bottom deflector 69 closer to the work rolls 25, and alsoproviding for particular angular contact of the descaling fluid to thesteel strip 12. Any well-known system may be used for controllinglongitudinal movement of the entry guide rolls 53 or the pivotalmovement of the descaling hood 63 such as electromechanical, pneumaticor hydraulic systems.

I claim:
 1. A system for minimizing the thickness of an oxide layer onfinished strips rolled in a hot rolling mill, said system comprising thesteps of:centering the head ends of said strips on a delay table as theyleave a roughing stage of said mill; maintaining the head ends of saidstrips on the center of said delay table as said strips enter afinishing stage of the mill by (1) freely allowing cambered areas ofsaid strips to veer off a centered alignment on said delay table, and(2) realigning said head ends before they enter said finishing stagewhile not attempting alignment of said cambered areas of said strips;descaling said strips at a location on said delay table that isdownstream of all alignments or realignments of said heads; andcollecting said descaling spray and mixed-in scales as they rebound offthe surfaces of said strips.
 2. A method for implementing the system setforth in claim 1 in an existing hot rolling mill having an entry guideimmediately adjacent to and upstream from the work rolls of a first millstand in a finishing mill, said method comprising the steps of:removingsaid entry guide; and inserting a descaler in place of said entry guide.3. A method and system as set forth in claim 2 wherein said systemincludes the steps of:delivering fluid to the top and bottom surfaces ofsaid strips by said descaler such that the velocity of the fluid issufficient to remove a surface oxide layer from each of said surfaces ofsaid strips, said fluid impacting on the top and bottom surfaces of saidstrips at an angle with respect to a plane parallel to said top andbottom surfaces such that oxide scales removed by and mixed with saidfluid are virtually all carried upstream with respect to the directionof movement of said strips, thereby eliminating any need for dammingrolls downstream of said descaler.
 4. A method for modifying theapparatus associated with a delay table connecting two adjacent ills ina rolling mill, said method comprising the steps of:removing an entryguide located upstream and immediately adjacent to the work rolls of afinishing mill; and inserting a descaler in place of said entry guide.5. In a hot rolling mill modified in accordance with claim 11, a methodfor providing an improved finishing operation by minimizing the oxidelayer formed on the surface of elongated steel strips as they traveldown a delay table toward a pair of working rolls, said methodcomprising the steps of:delivering fluid to the top and bottom surfacesof said elongated steel strips such that the velocity of the fluid issufficient to remove a surface oxide layer from each of said surfaces ofsaid strips, said fluid impacting on the top and bottom surfaces of saidstrips at an angle with respect to a plane parallel to said top andbottom surfaces such that oxide scales removed by and mixed with saidfluid are virtually all carried upstream with respect to the directionof movement of said strips; and deflecting said fluid from the surfacesof the elongated steel strips after said fluid impacts on said surfacesso as to direct said fluid and said scales mixed therein away fromupstream portions of said elongated steel strips, thereby preventingre-depositing of said scales on said strips at an area upstream from thearea of fluid impact.
 6. A method as set forth in claim 5 including thestep of:centering each of said strips at a predetermined distanceupstream from said pair of working rolls by centering the head end ofeach strip on a delay table and maintaining the head end proximate thecenter of said delay table by allowing the central portion of a camberedstrip to move away from the center of said delay table.